In recent years, the use of thermal ink jet printers in numerous applications has increased dramatically. Such printers make use of liquid-based inks which are sprayed onto a receptor, typically a sheet of paper or film, to produce an image. By using four basic ink colors (black, yellow, cyan, and magenta) in various combinations and concentrations, virtually any color may be produced as part of the printed image. Additionally, ink jet technology is well-suited for high resolution graphic images, particularly those produced using electronic printing systems. Such systems typically employ computer technology to create, modify, and store images, text, graphics and the like.
Many of the inks that have been used in the past with ink jet and other printers are primarily comprised of dyes contained within organic- or water-based carrier liquids. Although such inks may offer satisfactory performance in certain applications, the present trend is away from such systems, since such systems tend to produce images that lack the light stability and durability required for outdoor and similarly demanding applications. Additionally, the use of organic-based carrier liquids creates numerous environmental and material-handling complications. Rather, the printing industry has sought inks which are primarily water-based, to thereby reduce or eliminate the problems associated with organic solvent-based systems.
Initial attempts to form water-based pigmented inks for ink jet applications were less than successful. In one approach, such inks comprised a suspension of pigment particles in a water-based carrier. Unfortunately, the suspended pigments tended to agglomerate. Since ink jet printers make use of very small jet nozzles (on the order of less than about 80 micrometers) to provide high resolution images, the resulting pigment agglomerations had a tendency to restrict or clog the printer heads. This effect is referred to herein as "plugging". Additionally, in the case of thermal ink jet systems, such inks also suffered from the tendency of materials to settle onto, and coat, the heating elements of the printer heads. This causes a decreased thermal efficiency of the print head which results in the formation of smaller ink droplets and lower image quality. This effect is commonly referred to as "kogation".
To overcome the problems described above, some water-based ink jet inks have employed dispersants. In one approach, the dispersants were formed from surfactants which contain a hydrophilic portion as well as a hydrophobic portion. In another approach, copolymers having hydrophilic segments and hydrophobic segments were used. Examples of these approaches are described in the art, for example in U.S. Pat. No. 4,597,794 and U.S. Pat. No. 5,085,698.
In the approaches described above, the hydrophobic segments of the surfactant or polymer can absorb onto pigments by "hydrophobic" interaction between the dispersant molecule and the organic-based pigments whose surfaces tend to be hydrophobic. This hydrophobic interaction is usually not very strong. As a result, in thermal ink jet systems, there is a possibility that the dispersant molecules can desorb from the pigment surface, thereby allowing the pigment particles to flocculate. During printing, this can result in plugging of the printer head jet nozzles.
Other attempts involving the use of polymeric dispersants have been attempted as well. For example, water-soluble polymers and particulate polymer dispersions have also been considered, yet these have yielded only partial success. In particular, such systems, although promising, have tended to produce non-uniform printed solid block images. The lack of uniformity in the printed image, which becomes more pronounced with prolonged printing, is caused by incomplete coverage of the receptor surface in the image area. This problem, commonly referred to as "banding" results from progressively smaller projected ink drops over the course of a printing job. This effect is believed to be a result of kogation, caused by deposition of thermal insulating materials on the heating elements within the printing cartridge. As a result, heat transfer efficiency into the ink is reduced, thereby reducing the ability to produce properly sized ink bubbles needed for the printing process. Even if the deposited material is thermally conductive, it may still change the nucleation behavior on the heater surface during heating which also may adversely affect the bubble formation.
In addition, there is some reason to believe that the presence of any residual polymeric material which is not adsorbed on a pigment particle may have a propensity to deposit onto the heater elements when they are intensely heated during the jetting process. It may also be possible that the polymeric dispersant molecules which are adsorbed on pigment particles may increase the adhesion of pigment particles to the heater elements at elevated temperatures. The detrimental effects of deposition of foreign substances on the heater elements, either polymeric dispersant or pigment particles, are kogation and the jetting problems described above.